1. Field of the Invention
The present invention relates to a method of manufacturing an MIS (Metal-Insulator Semiconductor)-type semiconductor device, and more particularly relates to a method of manufacturing an MIS-type semiconductor device having a GaAs semiconductor substrate.
2. Description of the Prior Art
For the manufacturing of an MIS-type semiconductor device, a gate insulating film is first formed on a GaAs semiconductor substrate (hereinafter referred to as a "GaAs substrate") and a gate electrode is then formed on the gate insulating film. For the formation of the gate insulating film, a film of SiO.sub.2, SiN, AlN, or the like, is directly deposited on the GaAs substrate. Alternatively, the surface of the GaAs substrate is oxidized by plasma oxidation or anodic oxidation to form a gate insulating film thereon.
In the operation of an MIS-type semiconductor device manufactured by a conventional method, however, it is difficult to form an inversion layer in the surface portion of the GaAs substrate. This is because the density of the state generated in the interface (interface state) between the GaAs substrate and the insulating film is high (e.g., 10.sup.12 cm.sup.-2 /eV or higher). In the surface portion of the GaAs substrate, some positions of Ga in the lattice are occupied by As (As.sub.Ga) instead of Ga and some positions of As in the lattice are occupied by Ga (Ga.sub.As) instead of As (such As or Ga atoms in the wrong positions of lattice are called anti-site defect). This causes a high-density state in the surface portion of the GaAs substrate.
FIG. 7 shows an energy band diagram of the surface portion of a GaAs substrate. The diagram shows the As.sub.Ga donor state (energy level of 0.65 eV) caused by As atoms in Ga-sites of the lattice and the Ga.sub.As acceptor state (energy level of 1.1 eV) caused by Ga atoms in As-sites of the lattice.
In an MIS-type semiconductor device having an AlGaAs film used as a gate insulating film, there is little possibility that a high-density interface state (surface state) is generated. The interface state density of this MIS-type semiconductor device is as low as that of an MIS-type semiconductor device having a silicon semiconductor substrate. The energy band gap of the gate insulating film made of AlGaAs, however, is not more than about 2 eV, which causes a possibility that the gate insulating film will conduct current when a high bias voltage is applied to the MIS-type semiconductor device.